1. Field of the Invention
This invention relates to structures of thin film magnetic write heads. More specifically, the invention relates to control methods and structures of thin film write heads for thermally assisted recording, having integrated thermal fly height control.
2. Description of the Related Art
The ongoing quest for higher storage bit densities in magnetic media used in, for example, hard disk drives, have reduced the size (volume) of data cells to the point where the cell dimensions are on the order of the grain size of the magnetic material. For cells this small, there is concern that data stored within the cells is no longer thermally stable over long periods of time, as random thermal fluctuations at ambient temperatures are sufficient to erase data. This state is described as the superparamagnetic limit, which determines the maximum theoretical storage density for a given magnetic media. This limit may be raised by increasing the coercivity of the magnetic media or lowering the temperature. Lowering the temperature is not a practical option when designing hard disk drives for commercial and consumer use. Raising the coercivity is a practical solution, but requires write heads employing higher magnetic moment materials, or techniques such as perpendicular recording (or both).
One additional solution has been proposed, which employs heat to lower the effective coercivity of a localized region on the magnetic media surface; writes data within this heated region, where the local coercivity is lower, with a broad (and somewhat lower strength) magnetic field; and, stores the data state by cooling the media to ambient temperatures, where the coercivity returns to the quiescent high value. This technique is broadly referred to as “thermally assisted (magnetic) recording”, TAR or TAMR. It can be applied to both longitudinal or perpendicular recording systems, although the highest density state of the art storage systems are more likely to be perpendicular recording systems. Heating of the media surface is accomplished by a number of techniques such as focused laser beams or near field optical sources.
Another important feature integrated into today's magnetic heads includes a heating element and control system to adjust the fly height of the head. This is done by thermally deforming the head, via thermal expansion, in a controlled manner to fine tune the actual position of the ABS (air bearing surface) relative to the media surface. Typical heads undergoing this thermal deformation are shown in FIGS. 1 and 2 (Prior Art).
FIG. 1 (Prior Art) is a partial cross sectional view 100 of a typical thin film longitudinal head with thermal fly height control (TFC), wherein the write head comprises write gap 112 bounded by upper 110 and lower 108 pole tips. Upper pole tip 110 is in contact with upper return pole layer 114. Lower pole tip 108 is in contact with lower return pole layer 106. Lower return pole layer is separated from shield layer 102 by insulating layer 104. The coil is shown as structure 116, embedded in insulating layer 118, which may also be referred to as an overcoat layer. The read head comprises a MR (magneto-resistive) sensor 103 located between upper 102 and lower 101 shield layers. Shield layer 101 is supported by undercoat layer 120 and an AlTiC base layer 122. A thermal heater (not shown) is utilized to heat the head. The heating causes thermal expansion of the head components, which results in movement of the air bearing surface (ABS) toward the media, reducing the effective fly height (or the head to media clearance). The dashed line labeled ABSc in FIG. 1 represents the position of the air bearing surface of the head unheated, whereas the dashed line ABSh represents the position of the air bearing surface when heated. The magnitude of the expansion effect can be controlled by the amount of heat added and the resulting temperature of the head components, effectively adjusting the fly-height to the desired level. Lowering of the fly height is generally desirable during the write process, but may not be required for reading data or moving from one sector to another. In the latter cases, it is desirable to increase the fly height to reduce the potential to contact asperities on the media surface which could cause head damage.
FIG. 2 (Prior Art) is a partial, cross sectional view 200 of a typical thin film perpendicular head with thermal fly height control (TFC). The head comprises shield layers 202, 204, MR sensor 203, shaping layer 210, coil structure 208, main pole 212, lower return pole layer 206, wrap around shield 214, and upper return pole layer 216. Alternatively, structure 214 may also be a trailing shield. Shield layer 202 is supported by undercoat layer 218 and an AlTiC base layer 220. Details of wrap around shields and trailing shields, as applied to perpendicular recording heads, can be found in, for example, US Patent Application Publications 2007/0146930, 2007/0115584, 2006/0174474, 2006/0044682, and 2007/0137027. The dashed line labeled ABSc in FIG. 2 represents the position of the air bearing surface of the head unheated, whereas the dashed line ABSh represents the position of the air bearing surface when heated. As with the longitudinal head described above, the magnitude of the expansion effect can be controlled by the amount of heat added and the resulting temperature of the head components, effectively adjusting the fly height to the desired level.
Conventional heads, such as those shown in FIGS. 1 and 2 (Prior Art), having thermal flight control (TFC) systems usually employ resistive heaters imbedded within the head structure, which depend on thermal conduction to apply heat to the critical read and write structures at the ABS. One difficulty in this approach results from the transient delays that occur when power levels to the TFC heaters are changed to heat or cool the head in response to desired fly height corrections. For example, it is often required to reduce the fly height in zones where writing of data is desired. Lowering the fly height requires heating the head to increase the temperature. However, simply increasing the TFC heater power level to it's nominal steady state value may not cause the desired temperature change to occur fast enough, due to thermal delays in the head structure. A solution has been proposed in US patent application publication 20050057841A1, wherein a power pulse is applied to the TFC heater immediately prior to the data write zone. This process is illustrated in FIG. 7 (Prior Art).
FIG. 7 (Prior Art) is a sequence of charts 700, 702, 704, and 706 illustrating write coil power, TFC power, head temperature, and fly height as a function of time during a data write cycle. Chart 700 shows average write coil power for data being written between times t2 and t3 as the head flies over the media surface. The charts shows normalized average power. The actual average power will vary from sector to sector, depending on the duty cycle and profile of the data written. The instantaneous coil power levels are not shown for simplicity, as the detail is unnecessary when considering the heat generated by the coil during any write cycle. Chart 702 shows the TFC power levels. A power level of P0 is used to obtain a temperature of T1 in the head, resulting in a fly height of H2 (see charts 704, 706). At time t1, a pulse of power P1 is applied to the TFC heaters to shorten the response time of the heating process. The pulse is terminated as the coil power comes on at t2. The pulse combined with the coil power raises the head temperature to T2 (chart 704) and lowers the fly height from H2 to H1 during time period t2 to t3, as data is being written.
The difficulty with the proposed process of FIG. 7 (Prior Art) is that while it reduces the transient response time somewhat, it really does not address the thermal delays due to the location and thermal mass of the heaters. Furthermore, applying high power pulses to a system with high thermal inertia can create thermal oscillation and control difficulties. Adding TAR heat sources further complicate the thermal management of the head, as these sources will be on only during write cycles.
What is needed is an improved method for thermal fly height control with write heads having thermally assisted recording.